1. Field of the Invention
The present invention relates to a disc drive apparatus capable of executing recording and playback operations on a disc-shaped recording medium.
2. Description of Related Art
As disc-shaped recording media, optical discs, magneto-optic discs (MO discs), phase change discs (PC discs) and so forth have been developed.
Whereas optical discs used in various fields, such as CDs (compact discs), are required to be 1.2 mm in thickness, along with the size contraction of discs in recent years, magneto-optic discs known as HS (hyper storage) discs are required to be 0.8 mm in thickness, and DVDs (digital versatile discs/digital video discs), which are suitable for multimedia use, are required to be 0.6 mm in thickness. Furthermore, development of disc-shaped recording media of 0.1 mm or 0.01 mm in disc thickness is under way.
However, if the thickness of a disc-shaped recording medium is made thinner than 1.2 mm, the influence of dust or the like sticking to the disc surface increases, and gives rise to the problem that it is made difficult for the disc drive apparatus to execute accurate recording and playback operations.
Hereupon, with reference to FIGS. 1A and 1B and FIG. 2, explanation will be made of reading errors which may be invited by the influence of disc thickness and dust stuck to the disc when playback is executed by a conventional disc drive apparatus.
FIG. 1 illustrates the waveforms of reproduced RF signals obtained by a disc drive apparatus when dust of a certain magnitude is stuck to the surface of a disc, FIG. 1A showing the waveform of a reproduced RF signal obtained when dust of a certain magnitude is stuck to the surface of a disc having a thickness of 1.2 mm. In this case, though the level of the reproduced RF signal is somewhat reduced by the influence of the dust stuck to the surface of the disc, no level drop heavy enough to invite a reading error in the disc drive apparatus occurred.
On the other hand, FIG. 1B shows the waveform of a reproduced RF signal obtained by the disc drive apparatus when dust of the same magnitude as in FIG. 1A is stuck to the surface of a disc having a thickness of, for example, 0.6 mm. In this case, as the cross section of the light beam coming incident from the disc drive apparatus on the disc becomes smaller and, where dust or the like of the same size as in FIG. 1A is stuck to the surface of the disc, the proportion of the dust per unit area becomes greater, the level of the reproduced RF signal obtained by the disc drive apparatus steeply drops in a short period of time under the influence of the dust sticking to the disc surface, resulting in a situation in which the disc drive apparatus is highly likely to commit a reading error.
FIG. 2 illustrates the relationship between the thickness of the disc and the size of dust stuck to the disc on the one hand and the quantity of error propagation in the disc drive apparatus on the other. Incidentally, in FIG. 2, a drop in the level of a reproduced RF signal obtained by the disc drive apparatus to 55% or less of the normal level when no dust or the like is stuck is supposed to be an error level. To add, this error level is no more than an example, and the error level is supposed to differ from one disc drive apparatus system to another.
It is seen from this FIG. 2 that, even when the size of dust stuck to the surface of the disc is small, reading errors become more likely to occur as the thickness of the disc decreases from 1.2 mm to 0.60 mm, 0.30 mm, 0.15 mm and 0.02 mm.
Then, in order to prevent the influence of dust or the like stuck to the surface of disc from inviting a drop in the level of reproduced RF signals obtained by a disc drive apparatus even when the thickness of the disc is reduced, it is conceivable to provide the disc drive apparatus with an auto gain control (AGC) circuit to amplify the reproduced RF signals obtained by the disc drive apparatus to a prescribed level.
However, there was the problem that, if the disc drive apparatus is provided with an AGC circuit and the reproduced RF signals are amplified to a prescribed level, as the AGC circuit would also amplify amplifier noise and the like contained in the reproduced RF signals, the carrier-to-noise (C/N) ratio would deteriorate.
Also, it was difficult to correct the recording level with the AGC circuit during the recording operation of the disc drive apparatus.